Titanium silicide is commonly incorporated into semiconductor devices. Titanium silicide has advantages of low resistivity, the ability to be self-aligned, and reasonable thermal stability. However, the polymorphic characteristics of titanium silicide create various problems. For instance, titanium silicide (TiSi2) typically exists in either an orthorhombic base-centered phase having 12 atoms per unit cell and a resistivity of about 60 to 90 micro-ohm-cm (which is known as the C49 phase), or as a more thermodynamically-favored orthorhombic face-centered phase which has 24 atoms per unit cell and a resistivity of about 12-20 micro-ohm-cm (which is known as the C54 phase). It is common for the less-desired C49 phase to be initially deposited under typical processing conditions. The C49 phase is then converted to the desired C54 phase through an appropriate high-temperature annealing step. However, the annealing step can have detrimental effects on the silicide and other integrated circuit elements, particularly at small line-widths.
An exemplary set of processing conditions for forming titanium silicide includes: (1) pre-cleaning of a surface onto which the silicide is to be formed, (2) titanium deposition, (3) silicide formation at a temperature of less than about 700° C., (4) selective patterning of the silicide, and (5) phase transformation of the silicide from the C49 phase to the C54 phase at a temperature greater than about 700° C. The temperature utilized for the conversion of the C49 phase to the C54 phase is typically at least 50° C. above the formation temperature of the C54 phase in order to insure full transformation to the C54 phase.
A possible reason for the C49 phase forming first is because such has a lower surface energy than the C54 phase, and the higher surface energy of the C54 phase creates a significant energy barrier to formation of the C54 phase. The annealing of titanium silicide in the C49 phase overcomes the energy barrier associated with formation of the C54 phase, and thus enables growth of the C54 phase. A problem with the transformation from the C49 phase to the C54 phase is agglomeration. If the thermal energy utilized to obtain the phase transformation is excessive, a morphological degradation of the titanium suicide can result, resulting in agglomeration of the silicide. As the amount of silicide decreases due to decreasing line-widths and decreasing thicknesses of silicide film, the thermal energy needed to cause the transformation from the C49 phase to the C54 phase increases, and yet the thermal energy level at which the silicide film starts to agglomerate decreases. Thus, the process window for performing the phase transformation from the C49 phase to the C54 phase is narrowed, creating increasingly stringent tolerances for control of the transformation from the C49 phase to the C54 phase in order to avoid undesired agglomeration.
The processing conditions described above for formation of titanium silicide often involve formation of titanium directly on silicon, and subsequent formation of silicide from reaction of the titanium with the silicon. Recently, it was found that agglomeration problems can be reduced if a thin film of refractory metal is provided between the titanium and the silicon prior to formation of titanium silicide. Suitable refractory metals are those which are capable of forming metal silicide, and can include, for example, molybdenum, tungsten, tantalum, niobium, vanadium, and chromium, as discussed in, for example, U.S. Pat. No. 6,187,679. However, there can be difficulties in utilizing such methodologies in that if the layer of refractory metal is not kept very thin there will frequently not be suitable formation of titanium silicide over the refractory metal. Specifically, the titanium silicide will either not entirely convert to TiSi2 (instead the titanium silicide will have a composition of TiSix, with x being greater than 0 and less than 2) and/or the titanium silicide will not entirely convert to the desired C54 phase. Accordingly, it is desired to develop improved methods for forming titanium silicide. Also, even though titanium silicide is currently a commonly utilized silicide in semiconductor devices, there are other silicides which can have suitable characteristics for incorporation into various semiconductor devices, and it would be desirable if the methodologies developed for improved formation of titanium silicide could be extended to suicides other than titanium silicide.